Method for producing an aqueous hydroxylamine solution devoid of salt

ABSTRACT

A process for preparing a salt-free aqueous hydroxylamine solution by reacting an aqueous solution of a hydroxylammonium salt with a base to give a mixture and separating from said mixture by distillation a salt-free aqueous hydroxylamine solution, involving using as a base an aqueous solution of a mixture of NaOH and KOH in a molar ratio Na + :K +  in the range from 70:30 to 95:5 and with a total concentration of Na +  and K +  in the range from 0.1 to 10 m/m % based on the total amount of the mixture.

The present invention relates to a process for preparing a salt-freeaqueous hydroxylamine solution by reacting an aqueous solution of ahydroxylammonium salt with a base to give a mixture and separating fromsaid mixture by distillation a salt-free aqueous hydroxylamine solution,which comprises using as a base an aqueous solution of a mixture of NaOHand KOH in a molar ratio Na⁺:K⁺ in the range from 70:30 to 95:5 and witha total concentration of Na⁺ and K⁺ in the range from 0.1 to 10 m/m %based on the total amount of the mixture.

High-purity concentrated aqueous hydroxylamine solutions are used interalia in the electronics industry, in conjunction for example with othersubstances for cleaning printed circuit boards or silicon wafers. Foruse in the electronics industry, concentrations of impurities—metal ionsin particular—of well below 1 ppm are normally required, i.e.,electronic grade product. The purity requirements imposed on the aqueoushydroxylamine solutions are increasing continually.

Industrially, hydroxylamine is produced as a hydroxylammonium salt,normally as hydroxylammonium sulfate. To prepare salt-free aqueoushydroxylamine solutions, a base is added to an aqueous solution of ahydroxylammonium salt and an aqueous hydroxylamine solution is separatedfrom the mixture, normally by distillation, in accordance for examplewith U.S. Pat. No. 5,472,679, WO 97/22551, WO 98/57886, DE 1954775.8, WO99/07637.

The distillation of aqueous solutions containing hydroxylamine, even ona laboratory scale, is regarded as a particularly hazardous operation:see Roth-Weller: Gefährliche Chemische Reaktionen, StoffinformationenHydroxylamin, page 3, 1984, 2, Eco-med-Verlag.

Accordingly, the aforementioned distillation necessitates a high levelof technical complexity and a large amount of time.

Moreover, despite the distillation, aqueous hydroxylamine solutionscontain impurities from their preparation, such as sodium sulfate orother metal compounds, in undesiredly large amounts.

It is an object of the present invention to provide a process forpreparing salt-free aqueous hydroxylamine solutions by distillationwherein the aqueous hydroxylamine solutions are obtained with a lowlevel of impurities without any increase in technical complexity, timeor safety risk.

We have found that this object is achieved by the process defined at theoutset.

The process of the invention uses an aqueous solution of ahydroxylammonium salt and of a base. The solution may contain furthersubstances without deleterious effect on the process of the invention,such as hydroxylamine, or stabilizer, such as1,2-trans-(N,N,N′,N′-tetraacetic acid)-cyclohexanediamine or its salts,sodium or ammonium salts for example, or the stabilizers described in WO97/22551.

Suitable hydroxylammonium salts include salts of organic acids, such asformic acid, acetic acid, preferably salts of inorganic acids, such assulfuric acid, phosphoric acid, hydrochloric acid, or mixtures of suchsalts.

These hydroxylammonium salts and their preparation are known per se.

To prepare the solution of a hydroxylammonium salt and of a base, thehydroxylammonium salts may be used advantageously in the form of aqueoussolutions having a hydroxylammonium salt content of from 5 to 50 m/m %,in particular from 26 to 38 m/m %.

In accordance with the invention, the base used comprises an aqueoussolution of a mixture of NaOH and KOH in a molar ratio Na⁺:K⁺ in therange from 70:30 to 95:5, preferably from 80:20 to 90:10, in particularfrom 84:16 to 86:14. A suitable total concentration of Na⁺ and K⁺ is inthe range from 0.1 to 10 m/m %, preferably from 2.0 to 4.6 m/m %, and inparticular from 4.1 to 4.3 m/m %, based on the total amount of themixture.

This mixture may comprise further oxides or hydroxides of the alkalimetals, such as lithium hydroxide, of the alkaline earth metals, such ascalcium hydroxide, strontium hydroxide, barium hydroxide, ammonia,amines, such as monoamines, diamines or triamines, examples beingmethylamine, dimethylamine, trimethylamine, ethylamine, diethylamine,triethylamine, mono-, di- or trialkanolamines, such as diethanolamine,and cyclic amines, such as pyrrolidine or piperidine, and also mixturesof such bases. Advantageously, the mixture contains no further suchcompounds.

The base may be used advantageously in the form of an aqueous solution,preferably at a concentration of from 25 to 60 m/m %, in particular from45 to 50 m/m %.

The amount of base should be chosen so that the hydroxylammonium salt isconverted completely or at least predominantly into free hydroxylamine.This can be done continuously or batchwise and at temperatures in therange from about 10° C. to 120° C.

The aqueous solution of a hydroxylammonium salt and of a base that isused in the process of the invention should have a hydroxylamine contentof from 2 to 45 m/m %, preferably from 8 to 12 m/m %.

The base may be added during the distillation. It is advantageous to addthe base prior to the distillation.

The addition may be made continuously or batchwise and at temperaturesin the range between the melting point of the hydroxylammonium saltsolution and its boiling point, advantageously from about 0° C. to 100°C. Depending on the nature and concentration of the hydroxylammoniumsalt and on the reaction conditions employed for the liberation, such asthe temperature at which the reaction is conducted, some of the saltformed from the base cation and the acid anion present in thehydroxylammonium salt may undergo precipitation. If desired, thesolution may also be cooled in order to precipitate a larger amount ofthe salt. The reaction conditions and concentrations shouldadvantageously be chosen so that no salt is precipitated.

For the distillation for preparing the salt-free aqueous hydroxylaminesolution it is possible to contemplate customary single-stage ormultistage apparatus (stripping columns), such as are described, forexample, in Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd. Ed.,Vol. 7, John Wiley & Sons, New York, 1979, pages 870–881, such asevaporation chambers or rectifying columns, examples being sieve traycolumns, bubblecap tray columns, orderedly packed columns or randomlypacked columns.

Single-stage distillation chambers, pure stripping columns or rectifyingcolumns with stripping and enriching sections may be used here.

Suitable ordered or random packings include conventional packings ofthis kind, such as Raschig rings, Pall rings, and saddles.

The columns advantageously have a theoretical plate number in the rangefrom 5 to 70. The reaction mixture, to which a stabilizer may have beenadded, may be supplied directly to the top of the column (top section ofthe packing or topmost tray).

In the stripping column, the solution is fractionated in such a way thatthe salt fraction is taken off at the bottom of the column and anaqueous hydroxylamine fraction is taken off at the level of the feedtray or above, in particular from the column top. In order to achievethis it is preferred to treat the solution by passing steam and/or waterinto the column bottom in countercurrent. At a hydroxylamineconcentration in the feed solution in the range from 2 to 45 m/m %, thevolume flow of water or steam is generally from 1 to 8 times, inparticular from 1 to 5 times, the amount of feed.

The temperature of the steam introduced is generally in the range from80 to 180° C. If desired, it is also possible to heat the column bottomas well. The temperatures prevailing at the column top depend on thepressure at which the column is operated. This pressure is generallyfrom 5 to 300 kPa, preferably from 50 to 300 kPa. With particularpreference, the column may be operated at a pressure in the range from50 to 150 kPa. This pressure refers to the pressure at the top of thetray column.

The temperatures at the column top are, accordingly, generally in therange from 80 to 130° C., preferably from 90 to 120° C. The temperatureof the steam that is introduced may be significantly higher, even 150°C., for example. Advantageously, however, it should not be so high thattoo much water is evaporated from the salt solution and the salt beginsto precipitate in the column bottom.

If desired, a droplet separator (demister) may be installed above thefeed tray or in the vapor takeoff in such a way that entrainment of thesalt by droplets is prevented.

In one advantageous embodiment, the salt-free aqueous hydroxylaminesolution may be prepared by distillation in a tray column having atleast 2 actual trays.

Advantageously, the column should have an actual tray number in therange from 10 to 60. Suitable trays include crossflow trays such assieve trays, valve trays, bubble cap trays and tunnel trays, ordual-flow trays, preferably sieve trays. The distance between traysshould be in the range from 200 to 900 mm, preferably from 300 to 600mm.

The column and trays may be manufactured from nonmetallic materials,such as glass, ceramic, plastics. This prevents the decompositioninitiated by metal ions. Surprisingly, however, it has been found thatthe column may also be manufactured from specific metallic materials,such as platinum, silver, zirconium, without any significant increase inthe level of hydroxylamine decomposition observed.

The return ratio in the rectifying section may advantageously beregulated in such a way that it lies within the range from 0.2 to 2.

In accordance with the invention, located above at least one tray of thetray column, over the cross section of the column, are conventionalrandom packings such as Raschig rings, Pall rings, saddles, modernhigh-performance random packings such as the Hiflow ring (from Rauschert(Steinwiesen, Germany)), Super-Raschig rings (from Raschig(Ludwigshafen, Germany)), Cascade-Mini rings (from Koch-Glitsch(Wichita, USA)), IMTP rings (from Norton (Akron, USA)) or Nutter rings(from Sulzer Chemtech (Wintherthur, Switzerland)) or structured packingssuch as Mellapak, Mellapak Plus or woven packings, preferably modernhigh-performance packings.

The packings should be inert toward the solution to be distilled,consisting for example of plastics or special metallic materials,preferably of perfluorinated polymers (e.g., TFM, PFA, Teflon).

The fill level of the packings between the trays should be from 50 to300 mm, preferably from 100 mm to 200 mm. The distance between the bedof packings and the tray above which the bed of packings is installed isbetween 0 and 600 mm, preferably from 100 mm to 300 mm. The distancebetween the bed of packings and the tray below which the bed of packingsis installed is from 0 to 300 mm, preferably from 30 to 100 mm.

The pressure in the tray column is generally from 5 to 200 kPa,preferably from 10 to 110 kPa. It is particularly preferred to operatethe tray column at a pressure in the range from 50 to 110 kPa and inparticular under atmospheric pressure. This pressure relates to thepressure at the top of the tray column.

The temperatures prevailing in the tray column depend on the pressure atwhich the tray column is operated. They are generally in the range from30 to 130° C., preferably from 80 to 130° C.

The energy required for distillation may advantageously be supplied byintroducing steam in the bottom region. The temperature of thisintroduced steam should generally be in the range from 80 to 180° C.

The aqueous solution of a hydroxylammonium salt and of a base that isused in the process of the invention may be supplied at the top of thetray column, at one of the upper trays or, advantageously, at one of themiddle trays. If desired, a device for separating entrained droplets,such as a demister, may be installed above the feed tray.

The bottom product obtained in the process of the invention is anaqueous solution comprising a salt of the anion of the hydroxylammoniumsalt and the cation of the base.

In one preferred embodiment, the tray column used may be a side offtakecolumn.

In this case, the salt-free aqueous hydroxylamine solution is withdrawnat the side offtake. Water is then generally obtained at the top of thecolumn.

The salt-free aqueous hydroxylamine solution obtained in accordance withthe invention generally has a hydroxylamine content of from 1 to 20 m/m%, preferably from 8 to 12 m/m %.

The salt-free aqueous hydroxylamine solution obtained by the process ofthe invention has a greater purity than a solution obtained by knowndistillation methods. Moreover, the residence time of the distillationmixture in the column is shorter, and hence the thermal load is lower,than with known methods. Additionally, for a given column size and thesame column holdup, the capacity of the column is increased. If thecolumn includes a demister, the demister is unburdened relative to aknown method.

The hydroxylamine solution obtained may, if desired, be concentrated ina distillation column. It may be advantageous to add a stabilizer priorto the distillation. The hydroxylamine solution may be fed inadvantageously at a height of approximately one third of the actual traynumber of the distillation column. At the column top, substantiallyhydroxylamine-free water is obtained and at the bottom a hydroxylaminesolution whose concentration is dependent on the distillationconditions.

In general, the distillation column is operated with a pressure in therange from 1 to 200 kPa (from 0.01 to 2 bar), preferably from 5 to 120kPa (from 0.05 to 1.2 bar), with particular preference from 30 to 110kPa (from 0.3 to 1.1 bar), the pressure referring in each case to thepressure at the top of the column. The greater the extent to which thehydroxylamine is to be concentrated, the more gentle (low pressure andlow temperature) the distillation must be. The distillation may takeplace continuously or batchwise.

The temperatures prevailing in the distillation column are dependent onthe pressure at which the distillation column is operated. They aregenerally in the range from 10 to 160° C., preferably from 60 to 120° C.

The water or vapors taken off at the top of the distillation column maybe recycled to the bottom of the column used in the process of theinvention, directly or following compression or superheating asstripping steam, or may be supplied as waste water to a waste watertreatment facility.

If desired, a device for deposition of entrained droplets, such as ademister, may be installed above the feed tray.

As the distillation column it is possible to use conventional columns ina conventional manner. As the distillation column it is advantageous touse a tray column having at least two trays.

The column should advantageously have an actual tray number in the rangefrom 4 to 50. Suitable trays include crossflow trays such as sievetrays, valve trays, bubble cap trays and tunnel trays or dual-flowtrays, preferably sieve trays. The distance between these trays shouldbe in the range from 200 to 900 mm, preferably from 300 to 600 mm.

The column and trays may be manufactured from nonmetallic materials,such as glass, ceramic, plastics. This prevents the decompositioninitiated by metal ions. Surprisingly, however, it has been found thatthe column may also be manufactured from specific metallic materials,such as platinum, silver, zirconium, without a significant increase inhydroxylamine decomposition being observed.

Advantageously, a falling film evaporator is used to heat the columnbottoms, although it is of course also possible to use other customarybottoms heaters, such as natural-circulation or forced-circulationevaporators, plate-type heat exchangers, etc.

The return ratio in the rectifying section may advantageously beregulated in such a way that it lies within the range from 0.2 to 2.

In accordance with the invention, located above at least one tray of thetray column, over the cross section of the column, are conventionalrandom packings such as Raschig rings, Pall rings, saddles, modernhigh-performance random packings such as the Hiflow ring (from Rauschert(Steinwiesen, Germany)), Super-Raschig rings (from Raschig(Ludwigshafen, Germany)), Cascade-Mini rings (from Koch-Glitsch(Wichita, USA)), IMTP rings (from Norton (Akron, USA) or Nutter rings(from Sulzer Chemtech (Wintherthur, Switzerland)) or structured packingssuch as Mellapak, Mellapak Plus or woven packings, preferably modernhigh-performance packings.

The packings should be inert toward the solution to be distilled,consisting for example of plastics or special metallic materials,preferably of perfluorinated polymers (e.g., TFM, PFA, Teflon).

The fill level of the packings between the trays should be from 50 to300 mm, preferably from 100 mm to 200 mm. The distance between the bedof packings and the tray above which the bed of packings is installed isbetween 0 and 600 mm, preferably from 100 mm to 300 mm. The distancebetween the bed of packings and the tray below which the bed of packingsis installed is from 0 to 300 mm, preferably from 30 to 100 mm.

The concentrated aqueous hydroxylamine solution, which preferably has ahydroxylamine content in the range from 20 to 60 m/m %, is generallyobtained as the bottom product.

EXAMPLE

A hydroxylammonium sulfate solution (32 m/m %; about 0.6 kg/h) wasadmixed with sodium hydroxide/potassium hydroxide (each 50 m/m %; about1.7 kg/h) in different proportions, according to Table 1, withhydroxylamine, sodium sulfate and potassium sulfate being liberated. Ina laboratory column, a hydroxylamine solution was stripped off from thissalt solution, at the top, with a constant amount of steam. The solutionwas analyzed for Na⁺ and K⁺ ions.

The result is summarized in Table 1 below.

TABLE 1 Na⁺ [mg/kg] K⁺ [mg/kg] NaOH/KOH in hydroxylamine inhydroxylamine [m/m] solution solution 100:0 2.5 0  90:10 1.7 0.2  80:201 0.4

The contamination of the HA solution by metal ions is surprisinglyreduced by neutralization with a mixture of potassium hydroxide andsodium hydroxide, so that the Na⁺ contamination can be reduced to 40%.The ponderal sum of Na⁺ and K⁺ ions was reduced to 57%.

1. A process for preparing a salt-free aqueous hydroxylamine solution which comprises reacting an aqueous solution of a hydroxylammonium salt with a base to give a mixture and separating from said mixture by distillation a salt-free aqueous hydroxylamine solution, wherein the base is an aqueous solution of a mixture of NaOH and KOH in a molar ratio Na⁺:K⁺ in the range from 70:30 to 95:5 and with a total concentration of Na⁺ and K⁺ in the range from 0.1 to 10 m/m % based on the total amount of the mixture.
 2. A process as claimed in claim 1, wherein the mixture contains NaOH and KOH in a molar ratio Na⁺:K⁺ in the range from 80:20 to 90:10.
 3. A process as claimed in claim 1, wherein the total concentration of Na⁺ and K⁺ is in the range from 2.0 to 4.6 m/m % based on the total amount of the mixture.
 4. A process as claimed in claim 1, wherein the distillation is conducted in a tray column having at least 10 actual trays.
 5. A process as claimed in claim 4, wherein random or structured packings are located above at least one tray of the tray column over the cross section of the column.
 6. A process as claimed in claim 5, wherein the tray column has from 10 to 80 actual trays.
 7. A process as claimed in claim 5, wherein packing elements used comprise random packings, high-performance random packings or structured packings.
 8. A process as claimed in claim 5, wherein packing elements used comprise conventional random packings, selected from the group consisting of Raschig rings, Pall rings, and saddles.
 9. A process as claimed in claim 5, wherein packing elements used comprise: conventional random packings selected from the group consisting of Raschig rings, Pall rings and saddles; high-performance random packings; or structured packings.
 10. A process as claimed in claim 4, wherein the temperature in the tray column is in the range from 80° C. to 130° C.
 11. A process as claimed in claim 4, wherein the tray column is a side offtake column.
 12. A process as claimed in claim 11, wherein the salt-free aqueous hydroxylamine solution is obtained at the side offtake.
 13. A process as claimed in claim 11, wherein water is obtained as the column top product.
 14. A process as claimed in claim 1, wherein the distillation bottom product is an aqueous solution comprising a salt of the anion of the hydroxylammonium salt and the cation of the base. 